In a high voltage direct current system, insulation detection is a mandatory function, and leakage current detection is an essential part of the insulation detection. In some approaches, a common leakage current detection method is a sensor detection method. In the sensor detection method, the principle of electromagnetic induction is used, and detection is performed using a sensor (for example, a Hall effect sensor). In an embodiment, a sensor used to simultaneously detect currents flowing through positive and negative buses, and when insulation of the system is normal, because the currents have equal values but opposite directions, magnetic fields passing through the sensor are canceled by each other, and a detected leakage current is 0. When an insulation fault occurs in the system, regardless of a current leakage from the positive bus or a current leakage from the negative bus, values of the currents flowing through the sensor are not equal. Therefore, a magnetic field is not zero and a value of a current can be calculated by detecting a voltage signal output by the sensor. According to the sensor detection method, a difference between currents flowing through the positive and negative buses is detected, and a value of a leakage current is calculated by performing electromagnetic conversion for the sensor. This type of sensor has high manufacturing costs and low sensitivity, and cannot perform detection when a leakage current has a relatively small value. Moreover, the sensor is sensitive to a magnetic field, and a false alarm easily occurs when the sensor is used in an environment with high interference intensity.
A block diagram of the principle of present leakage current detection is shown in FIG. 1, and mainly includes a battery, a rectification module 11, a load 12, and a sensor 13. Positive and negative buses both pass though the sensor 13, to supply power to the load 12. Normally, when insulation of a system is normal, there is no current leakage, and a current I1 flowing through the positive bus and a current I4 flowing through the negative bus have equal values but opposite directions. Subsequently, magnetic fields in the sensor that are generated by I1 and I4 are canceled by each other, and output of the sensor 13 is 0. When an insulation fault occurs in the system (e.g., in the positive bus) with a current leakage, assuming that a leakage current is I2 (where Rin is an equivalent ground resistor), a value of I1 is equal to a value of I2 plus a value of I3, thus, a magnetic field passing through the sensor 13 is not 0. Furthermore, a value of a leakage current can be calculated by detecting a voltage output by the sensor. In an existing leakage current detection technology, the electromagnetic principle is used, and a leakage current is detected using a sensor. However, since the sensor has a poor capability of detecting a small signal, sensitivity is low. Moreover, due to the sensor is sensitive to a magnetic field, a false alarm may easily occur when the sensor is used in an environment with high interference intensity, which further reducing system reliability.
Consequently, in the existing leakage current detection technology, a leakage current is detected using the electromagnetic principle, and a sensor for detecting a magnetic field has relatively low sensitivity. Therefore, it is very difficult for the sensor to detect the leakage current when a leakage current is relatively low, and a false alarm may be easily triggered when the sensor for detecting a magnetic field is applied to an environment with relatively strong magnetic field interference, thereby reducing system reliability.